Overview

The intent of this paper is to describe how oMGs handle extended periods without GPS reception, explain the configuration parameters involved, and how these parameters can be used to modify default behavior. 

Functionality

By default, oMGs perform soft resets of the GPS module every 15 minutes (900 seconds) when no GPS fix is available, and then perform hard resets of the GPS module after five consecutive soft resets (1 hr 15 min).  It is the hard reset which causes the oMG to purge its last known location information and ephemeris data (see the Further Detail section below for more information). After a hard reset, it can take several minutes for the oMG to determine which satellites are in the current sky view and rebuild its ephemeris database, during which time it is not possible to compute a GPS fix.

A configuration file named gps.yaml contains parameters that control the behavior during extended GPS unavailability; the parameter format is provided below with default values. The parameters are not shown in the oMG’s LCI so the gps.yaml configuration file must be edited manually on the oMM and then downloaded to oMGs through the oMM’s mass configuration feature. 

enableHardReset: yes
enableSoftReset: yes
noFixResetDelaySeconds: 900
softResetCountBeforeHardReset: 5 

Hard and/or soft resets can be disabled by setting the associated enable[Soft|Hard]Reset values to "no". It’s useful to keep in mind that hard resets are only performed as an escalation from soft resets, so disabling soft resets implicitly disables hard resets. The noFixResetDelaySeconds parameter governs the delay between resets and softResetCountBeforeHardReset indicates how many soft resets must occur before a hard reset is attempted. To minimize the impact of moving from non GPS coverage to GPS, the following options are available:

1. Disable hard reset. The down side is that in rare circumstances, a reboot may be required to recover GPS on the oMG.   
2. Extend the noFixResetDelaySeconds and/or increase the softResetCountBeforeHardReset value such that the product of the two values is longer than the maximum time an active duty vehicle (e.g. ambulance) is expected to remain under cover (i.e. does not have a clear view of the sky). For instance bumping softResetCountBeforeHardReset from 5 to 96 would ensure that at least 24 hours pass between hard reset attempts. If the vehicle is expected to have a clear view of the sky at least once a day, then such a setting would prevent hard resets in most cases. However, extending beyond four hours may not provide additional benefit as explained in the Further Detail section below.
3. Install GPS repeaters in stations that are used extensively so that vehicles can maintain a GPS lock while parked there.

Further Detail

Two types of data are purged during GPS module hard resets: almanac and ephemeris.

Almanac data is coarse-grained satellite position data used to predict which satellites will be visible in the sky from a given location, in order to accelerate acquisition. The oMG 2000 has a 12-channel GPS receiver and there are between 24 and 32 satellites in the GPS constellation at any given time so it only takes a few minutes to scan for all possible satellites in the absence of almanac data.

Ephemeris data is fine-grained satellite position data that is required to calculate a precise location fix. It is updated every two hours and is generally valid for up to four hours. It takes 18 to 30 seconds to download new ephemeris data from each satellite.
This means that if a vehicle is parked under cover for an extended period of time (less than four hours) it can take some time to compute a fix, regardless of whether or not the GPS receiver has been reset, because the receiver must locate at least three satellites and then process 18 to 30 seconds worth of information from each of them. Defining the parameters above so that hard resets occur after approximately four hours under cover, may be the best compromise between maintaining GPS position accuracy and protecting against potential GPS lockups.

GPS repeaters solve this problem, ensuring that the receiver has accurate almanac and ephemeris data at all times.